Backlight always turns off after 2 minutes of inactivity

Hello! Goofy question of the day: I've got a very nice 17" PowerBook, but the screen backlight without fail shuts off after about 2 minutes of inactivity. Again, I know it's only the backlight because it has a faint image. If I close the machine and open it up, the light comes back, then goes off again after 2 minutes of inactivity, and you have to either power off/on or close and open the machine to trigger it to turn on again. If you actively use the machine, it stays on indefinitely and you never notice an issue. Because this is not an intermittent problem and the backlight is reliable otherwise, I tend to think this is probably not a bad screen/inverter/cable, but more likely a sleep/reed switch/sensor issue of some type? It's got a fresh install of Leopard, and I've tried turning off all the power saving settings, so I know it's not an OS issue. I'm looking for ideas on where to start, given that this is not the typical "dead backlight, replace the inverter cable" problem. Thanks!

I tried that, and the external display stays on when the backlight goes. Like I mentioned, it's only the backlight that goes out, not the video image, i.e. I can see a faint image and the computer continues to operate when the backlight is dark. I don't deal with these machines everyday, so I don't know exactly what I'm looking for, but I'm wondering if it's the equivalent of a faulty reed switch?

Still did it booting from another drive, so it's not software. Actually, I just reset the pram and pmu, and it seems to be fixed! I'm accepting your answer because you reminded me to remember the simple stuff (which the resets certainly are). Thanks!

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